1. Field
This invention relates to directional couplers and more particularly to couplers that employ discrete windings on ferrite cores.
2. Prior Art
FIG. 1 shows the schematic diagram of a prior art directional coupler. FIG. 3 shows the typical location of the windings on a binocular ferrite core for the coupler shown in FIG. 1. This type of core is referred to as a binocular core because of its shape. See the side view in FIG. 3A. In FIG. 1, the input of the main through line is port 2, while the output of this line is port 3. The coupled line input port is 4 while the output of this line is port 5. A winding 6 is connected in series with the main line, while a winding 7 is connected in series with the coupled line. Winding 6 is coupled to a main line secondary winding 8, while winding 7 is coupled to the couple line secondary winding 9. Winding 8 is connected at one end to ground and at the other end to port 5, while winding 9 is connected at one end to ground and at the other end to port 3. Note that in FIGS. 2 and 3, all windings have a first and a second end. The first end being located to the left proximate the input ports 2 and 4 and the second end being located to the right.
In the operation of this coupler, a first signal placed on the main line at port 2 leaves at port 3 and will produce two effects on the coupled line. First, a portion of this first signal will flow from port 3 through coil 9 to ground, inducing a second signal in coil 7 and thus onto the coupled line. This will occur regardless of whether the first signal on the main line is applied to port 2 or port 3.
The second effect on the coupled line is the first signal on the main line passing through coil 6 induces a third signal in coil 8 which feeds this signal to port 5 and thus onto the coupled line. However, this third signal on the coupled line has a direction depending on whether the signal on the main line is fed into port 3 or port 2 because it goes through a transformer comprised of coils 6 and 8. In a transformer, the direction of the first signal in the primary dictates the direction of the signal in the secondary.
It can be seen from the above discussion that the second and third signals are placed onto the coupled line. The third signal is directional and its direction depends on the direction of the first signal on the main line. The second and third signals are set to be nearly equal to each other so that a first signal flowing in one direction on the main line will cause the two reflected signals on the coupled line to aid, while a signal flowing in the opposite direction on the main line will cause the two signals on the coupled lines to cancel.
This characteristic produces the directionality of the coupler, and provides a signal out of the coupled line only when the first signal flowing on the main line is flowing in a specific direction.
All the windings for the directional coupler may be wound on a single binocular core as shown in FIGS. 3 and 3A. In FIG. 3, a core 12, having a first hole 12A and a second hole 12B, contains a winding 8 in the first hole and a winding 9 in the second hole. These winding are broken into two portions. That is, winding 8 has two parts, referred to as subwindings 8A and 8B which are connected in series and which, when considered together, comprise the complete winding 8. As can be seen in FIG. 3A, subwinding 8A is typically wound on the side of the core, while subwinding 8B is typically wound on the top of the core. Note that FIG. 3A shows a side cross sectional view of the binocular core 12 and the hole 12A passes through the core in a longitudinal direction.
Winding 9 is similarly broken in to subwinding 9A and 9B and these windings are located on the side and top respectively of the second hole 12B. The primary for winding 8, which is winding 6 shown in FIG. 1, is placed in the first hole 12A, while winding 7, also shown in FIG. 1, is placed in the second hole. However, the windings 6 and 7 are not shown in FIG. 3 to more clearly show windings 8 and 9 which are important in explaining the operation of the present invention and how this invention differs from the prior art. A problem occurs with this prior art arrangement of the windings on the core. The coupling through the core falls off at high frequencies reducing the performance that can be achieved at high frequency. There is a need to provide a different core winding scheme to obtain improved flatness, directivity and VSWR at high frequencies especially above 1000 MHz.